Surgeons approaching the posterior fossa should be familiar with the extracranial anatomy. The posterior neck muscles can be divided into three overlapping layers. The superficial layer includes the sternocleidomastoid muscle and the splenius capitis muscle. The middle layer includes the longissimus capitis muscle and the semispinalis capitis muscle. A white or yellow soft tissue membrane that lies under the splenius capitis muscle covers the middle layer including the fat tissue, the veins, and the occipital artery. Defining this membrane is helpful in protecting the occipital artery. The deep layer consists of the rectus capitis major muscle, the obliquus superior capitis muscle, and the obliquus inferior capitis muscle. The suboccipital triangle is outlined by the three muscles in the deep layer. The triangle frames the third portion of the vertebral artery.
Key Steps
Step 1. Identification of mastoid tip and asterion (Fig. 18.1)
Step 2. Identification of splenius capitis muscle (Fig. 18.4)
Step 3. Identification of occipital artery (Fig. 18.7)
Step 4. Identification of obliquus superior muscle (Fig. 18.18)
Step 5. Identification of vertebral artery (Fig. 18.21)
Illustrated Steps with Commentary
Fig. 18.1 (Step 1) Palpable landmarks.The mastoid tip and posterior body of the mastoid are palpable. A depression can usually be palpated just below the asterion. The inion marks the midline. (EAC, external auditory canal; EOP, external occipital protuberance; INL, inferior nuchal line; SNL, superior nuchal line)Fig. 18.2 The posterior auricular vein.On removing the skin the posterior auricular vein and occipital vein are prominent. The posterior auricular vein passes behind the ear draining into the external jugular vein. (OA, occipital artery; PAV, posterior auricular vein)Fig. 18.3 The occipital belly of the occipitofrontalis muscle and the sternocleidomastoid muscle.The most superficial muscles encountered are the sternocleidomastoid and occipital belly of the occipitofrontalis muscle. The trapezius muscle is also superficial with its lateral edge approximately 2–3 cm medial to the medial edge of the sternocleidomastoid muscle. (GAN, greater auricular nerve; GON, great occipital nerve; OA, occipital artery; OM, occipitalis muscle; P, parotid gland; PAM, posterior auricular muscle; PAV, posterior auricular vein; SCM, sternocleidomastoid muscle)Fig. 18.4 (Step 2) Fascia of the splenius capitis muscle.The splenius capitis attaches to the skull just below the sternocleidomastoid and passes medially to the nuchal ligament. The muscle is seen in the gap between the sternocleidomastoid and the trapezius. (FSC, fascia of splenius capitis; GAN, greater auricular nerve; GON, great occipital nerve; OA, occipital artery; OM, occipitalis muscle; P, parotid gland; PAM, posterior auricular muscle; PAV, posterior auricular vein; SCM, sternocleidomastoid muscle)Fig. 18.5 The superficial layer.The scalp is removed to reveal the most superficial layer of muscles. The sternocleidomastoid, trapezius, splenius capitis, levator scapulae, occipital belly of occipitofrontalis muscles, greater auricular nerve, parotid gland, occipital artery, greater occipital nerve, occipital vein, and posterior auricular vein are seen. (GAN, greater auricular nerve; GON, great occipital nerve; LS, levator scapulae muscle; OA, occipital artery; OM, occipitalis muscle; PAV, posterior auricular vein; SCM, sternocleidomastoid muscle; SNL, superior nuchal line; SpC, splenius capitis muscle; Trap, trapezius muscle)Fig. 18.6 The greater auricular nerve.The greater auricular nerve runs obliquely across the sternocleidomastoid muscle. The nerve crosses the anterior margin of the sternocleidomastoid muscle approximately 3 cm below the mastoid tip. This nerve is available should a graft be needed. (GAN, greater auricular nerve; MT, mastoid tip; P, parotid gland; PAV, posterior auricular vein; SCM, sternocleidomastoid muscle)Fig. 18.7 (Step 3) The occipital artery and the greater occipital nerve.The occipital artery passes through the fascia at the corner formed by the posterior edge of the sternocleidomastoid muscle and the superior edge of the splenius capitis muscle. The greater occipital nerve emerges from the corner formed by the superior edge of the splenius capitis muscle, the anterior edge of the trapezius muscle, and the posterior edge of the semispinalis capitis muscle. (GON, great occipital nerve; OA, occipital artery; OM, occipitalis muscle; PAV, posterior auricular vein; SCM, sternocleidomastoid muscle; SpC, splenius capitis muscle)Fig. 18.8 The sternocleidomastoid, trapezius, splenius capitis, levator scapulae, and semispinalis capitis muscles.Raising the occipitalis muscle we see the asterion and the junction of the squamosal suture with the parietomastoid suture. (GAN, greater auricular nerve; GON, great occipital nerve; Lam, lamb-doid suture; LS, levator scapulae muscle; OA, occipital artery; P, parotid gland; PMS, parietomastoid suture; SCM, sternocleidomastoid muscle; SNL, superior nuchal line; SpC, splenius capitis muscle; Sq, squamosal suture; Trap, trapezius muscle)Fig. 18.9 The accessory nerve.The accessory nerve runs through the two bellies of sternocleidomastoid muscle. (P, parotid gland; SCM, sternocleidomastoid muscle; XI, accessory nerve)Fig. 18.10 The splenius capitis muscle.The splenius capitis muscle runs under the sternocleidomastoid muscle from the nuchal ligament and upper thoracic spinous processes to an attachment just below the sternocleidomastoid muscle. (LS, levator scapulae muscle; P, parotid gland; SpC, splenius capitis muscle; Trap, trapezius muscle)Fig. 18.11 Removing the sternocleidomastoid muscle.After the sternocleidomastoid muscle is removed, the splenius capitis muscle is seen. (DM, digastric muscle; IJV, internal jugular vein; LS, levator scapulae muscle; P, parotid gland; SpC, splenius capitis muscle; Trap, trapezius muscle; XI, accessory nerve)Fig. 18.12 Anatomy of the high cervical muscles.The semispinalis capitis originates from the transverse processes of the lower cervical and upper thoracic spine and inserts medially between the upper and lower nuchal lines. (DM, digastric muscle; IJV, internal jugular vein; LS, levator scapulae muscle; P, parotid gland; SpC, splenius capitis muscle; XI, accessory nerve)Fig. 18.13 The styloid diaphragm.The occipital artery and veins runs within the fatty tissue and connective tissue underneath the thick fascia known as the styloid diaphragm. (DM, digastric muscle; GON, great occipital nerve; IJV, internal jugular vein; LS, levator scapulae muscle; OA, occipital artery; P, parotid gland; Trap, trapezius muscle; XI, accessory nerve)Fig. 18.14 Removing the splenius capitis muscle.Removing the splenius capitis muscle reveals the longissimus capitis muscle, occipital artery, semispinalis capitis muscle, and posterior belly of the digastric muscle. In this case the occipital artery passes over the longissimus capitis muscle. In other cases the artery crosses under this muscle. (DM, digastric muscle; GON, great occipital nerve; IJV, internal jugular vein; LgC, longissimus capitis muscle; LS, levator scapulae muscle; OA, occipital artery; P, parotid gland; SsC, semispinalis capitis muscle; Trap, trapezius muscle; XI, accessory nerve)Fig. 18.15 Another variation of the course of the occipital artery.In this case the occipital artery passes under the longissimus capitis muscle. (LgC, longissimus capitis muscle; LS, levator scapulae muscle; OA, occipital artery; SsC, semispinalis capitis muscle)Fig. 18.16 The longissimus capitis muscle.The longissimus capitis muscle runs from the transverse processes of the lower cervical and upper thoracic vertebrae to insert under the mastoid process. (DM, digastric muscle; IJV, internal jugular vein; LgC, longissimus capitis muscle; LS, levator scapulae muscle; P, parotid gland; SsC, semispinalis capitis muscle; Trap, trapezius muscle)Fig. 18.17 The styloid diaphragm.The styloid diaphragm continues forward under the longissimus capitis muscle. The styloid diaphragm covers the deep layer of the posterior neck muscles. (DM, digastric muscle; IJV, internal jugular vein; LS, levator scapulae muscle; P, parotid gland; SsC, semispinalis capitis muscle; Trap, trapezius muscle)Fig. 18.18 (Step 4) Obliquus superior muscle.The obliquus superior muscle originates lateral to the semispinalis capitis muscle between the superior and inferior nuchal lines. It inserts into the upper surface of the transverse process of C1. (DM, digastric muscle; IJV, internal jugular vein; LS, levator scapulae muscle; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; P, parotid gland; RCL, rectus capitis lateralis muscle; SsC, semispinalis capitis muscle; Trap, trapezius muscle)Fig. 18.19 The obliquus superior, the obliquus inferior, and the rectus capitis major muscles.(DM, digastric muscle; IJV, internal jugular vein; LS, levator scapulae muscle; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; P, parotid gland; RCL, rectus capitis lateralis muscle; RCMjM, rectus capitis major muscle; Trap, trapezius muscle)Fig. 18.20 The suboccipital triangle.The suboccipital triangle is formed by the obliquus capitis superior, obliquus capitis inferior, and rectus capitis posterior major. (LS, levator scapulae muscle; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; RCL, rectus capitis lateralis muscle; RCMjM, rectus capitis major muscle; SOT, suboccipital triangle; TP, transverse process of the C1 [atlas])Fig. 18.21 (Step 5) Horizontal V3 portion of the vertebral artery in the suboccipital triangle.The horizontal segment of the vertebral artery lies in the suboccipital triangle. The rich venous plexus that accompanies this artery has been removed. (C1, C1 nerve; IJV, internal jugular vein; LS, levator scapulae muscle; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; PA, posterior arch of the C1 [atlas]; RCL, rectus capitis lateralis muscle; RCMjM, rectus capitis major muscle; TP, transverse process of the C1 [atlas]; VA, vertebral artery; XI, accessory nerve)Fig. 18.22 Magnified view of Fig. 18.21.The horizontal segment of the vertebral artery is well seen. The C1 nerve emerges from between the vertebral artery and the posterior arch of the C1 (see Fig. 18.28). (C1, C1 nerve; LS, levator scapulae muscle; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; PA, posterior arch of the C1 atlas); RCMjM, rectus capitis major muscle; VA, vertebral artery)Fig. 18.23 Muscle attachments to the transverse process.The rectus capitis lateralis, the obliquus superior, the obliquus inferior, the levator scapulae, and the scalenus medius are attached to the transverse process of C1. The levator scapulae covers the attachment of the splenius cervicalis. (LS, levator scapulae muscle; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; RCL, rectus capitis lateralis muscle; TP, transverse process of the C1 [atlas]; VA, vertebral artery; XI, accessory nerve)Fig. 18.24 Demonstrating the anatomy of the deep layer of occipital muscles.(DM, digastric muscle; IJV, internal jugular vein; LS, levator scapulae muscle; MT, mastoid tip; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; P, parotid gland; PA, posterior arch of the C1 [atlas]; RCL, rectus capitis lateralis muscle; RCMjM, rectus capitis major muscle; TP, transverse process of the C1 [atlas]; VA, vertebral artery)Fig. 18.25 Less magnified view of Fig. 18.24.The gross anatomy of retroauricular and high cervical region is well understood. (As, asterion; DM, digastric muscle; IJV, internal jugular vein; Lam, lambdoid suture; LS, levator scapulae muscle; MT, mastoid tip; OCIM, obliquus capitis inferior muscle; OCSM, obliquus capitis superior muscle; OMS, occipitomastoid suture; P, parotid gland; PMS, parietomastoid suture; RCL, rectus capitis lateralis muscle; RCMjM, rectus capitis major muscle; Sq, squamosal suture; TP, transverse process of the C1 [atlas]; VA, vertebral artery)Fig. 18.26 Schema of the lateral craniocervical junction.(C1, C1 nerve; C1C, C1 condyle; C2, C2 nerve; F, facet; HC, hypoglossal canal; IJV, internal jugular vein; JB, jugular bulb; JG, J-groove for the vertebral artery; JT, jugular tubercle; OC, occipital condyle; PA, posterior arch of the C1 [atlas]; SS, sigmoid sinus; TP, transverse process of the C1 [atlas]; VA, vertebral artery)Fig. 18.27 The vertebral artery and the C2 nerve.The posterior wall of the transverse foramen of the atlas is removed to expose the vertebral artery. (C2, C2 nerve; IJV, internal jugular vein; JB, jugular bulb; PA, posterior arch of the C1 [atlas]; SS, sigmoid sinus; VA, vertebral artery)Fig. 18.28 Variation of the J-groove of the atlas.The horizontal V3 portion of the vertebral artery usually travels over a shallow notch in the superior border of the posterior lamina of the atlas, often referred to as the J-groove. Two variations of the J-groove are a bony canal formation (this Figure) and a fibrous canal formation (Fig. 18.29). (C1, C1 nerve; PA, posterior arch of the C1 [atlas]; VA, vertebral artery)Fig. 18.29 Variation of the J-groove of the atlas.The vertebral artery is tethered to the J-groove by the posterior atlanto-occipital membrane, which can vary in its thickness. Bone spurs, ponticles, can arise from the edges of the J-groove and on occasion can completely encircle the vertebral artery. In addition, the J-groove is nearly flat in 25% of cases. (OCIM, obliquus capitis inferior muscle; PA, posterior arch of the C1 [atlas]; VA, vertebral artery)Fig. 18.30 Overview of the anatomy of the posterior skull base following total petrosectomy.(C7, C7 portion of the internal cartoid artery; ICA, internal carotid artery; IJV, internal jugular vein; IX, glossopharyngeal nerve; JB, jugular bulb; PA, posterior arch of the C1 [atlas]; SS, sigmoid sinus; VA, vertebral artery; VII, facial nerve; XI, accessory nerve; XII, hypoglossal nerve)Fig. 18.31 Overview of the posterior skull base anatomy after condylectomy.(C2, C2 nerve; PA, posterior arch of the C1 [atlas]; TP, transverse process of the C1 [atlas]; VA, vertebral artery)Fig. 18.32 Vertebral artery at the craniocervical junction and C1-C2 junction.(C1C, C1 condyle; C2, C2 nerve; JG, J-groove for the vertebral artery; OC, occipital condyle; PA, posterior arch of the C1 [atlas]; TP, transverse process of the C1 [atlas]; VA, vertebral artery)Fig. 18.33 Opening the transverse foramen of the atlas.(C1, C1 nerve; HC, hypoglossal canal; VA, vertebral artery)Fig. 18.34 Posterior translocation of the vertebral artery.(C1, C1 nerve; HC, hypoglossal canal; VA, vertebral artery)
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